Currently, in the United States, most petroleum is produced by the natural drive of subterranean gas or water, or by water or steam flooding using one or more injection wells. When the petroleum is produced by water-drive, the petroleum is typically coming from relatively low permeability zones with pore sizes on the order of 1-10 microns, while the water is pouring through the larger channels that have already been swept of oil; the ratio of the volume of water to that of oil drawn from the production well is commonly from 20 to 100 or more. The electricity costs for pumping up such a large volume of water along with the oil can make the cost of oil recovery prohibitive.
There have been numerous proposals aimed at overcoming this problem by blocking the larger subterranean channels to reduce the flow of water that is unaccompanied by oil. These techniques, however, typically employ conventional petroleum-derived polymers which are injected in a very dilute water solution to at least partially block the larger flow channels. Examples of such polymers are described in U.S. Pat. No. 4,776,398, issued to Chu et al. on Oct. 11, 1988; U.S. Pat. No. 4,926,943, issued on May 22, 1990, to Hoskin; U.S. Pat. No. 4,940,091, issued on Jul. 10, 1990, to Chu et al.; U.S. Pat. No. 4,787,451, issued on Nov. 29, 1988, to Mitchell; U.S. Pat. No. 4,941,533, issued on Jul. 17, 1990, to Buller et al.; and U.S. Pat. No. 4,903,766, issued on Feb. 27, 1990, to Chu. These patents describe the use of large petroleum-derived and microbe-derived polymer molecules with molecular weights in the millions. The polymers are typically very expensive to produce and so are used in very dilute aqueous solutions.
There are a number of additional problems which are created by use of these petroleum-based, large polymer molecules. For one, the molecules tend to shear on injection, thereby losing their effectiveness. Further, the divalent salts which are commonly present in the subterranean petroleum-bearing formations tend to cause many polymer molecules to curl, thereby decreasing their size and so the effectiveness of the polymer. Still further, the polymers are subject to chemical, thermal and biological degradation, for example oxidation, in situ, decreasing their effectiveness. Also, an injection water source of suitable purity for the polymer may not be available in the field, or the available water may require extensive treatment. There is also the consideration that if these polymers are back-produced they will contaminate the petroleum. Accordingly, the water-soluble polymer injection techniques had not been widely accepted as effective, economical means of controlling the permeability of petroleum-bearing subterranean formations.